Method of operating electromagnetic striking-tools by means of alternatng current.



L. scHfiLBR. METHOD OF OPERATING ELECTROMAGNETIC STBIKING TOOLS BY MEANS OF ALTEBNATING CURRENT.

APPLICATION FILED AUG. 30, 1912;

Patented Oct. 27, 19%

3 SHEETS-SHEET 1.

L. SGHULER.

METHOD OF OPERATING ELECTROMAGNETIC STRIKING TOOLS BY MEANS OF ALTEBNATING CURRENT.

APPLICATION FILED AUG. 30, 1912.

1, 1 1 5,25 1 Patented Oct. 27, 1914.

3 SHEETS-SHEET 2.

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%*-% IjZdg Leo 86/5/2761 L. SOHELER. METHOD OF OPERATING ELECTROMAGNETIC STRIKING TOOLS BY MEANS OF ALTERNATING CURRENT.

APPLICATION FILED AUG. 30, 1912.

1,1 15,251 Patented Oct. 27, 1914.

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specification of Letters Patent. Application filed August 30, 1912. Serial ID. 711,873-

Patented Oct. 2'7, 1914.

To all whom it may concern:

Be it known that 1, L110 Somiman, a citizen of the Empire of Germany, residing at Berlin-Lichterfelde-West, in the Empire of Germany, have invented a new and useful Method of Operating Electromagnetic Str1king-Tools by Means of Alternating Current,

' tion with re erence to the accompanying of which the following is a specification.

My invention relates to a new method of operating electromagnetic striking tools by means of alternating current with the aid of synchronous interrupters, whereby a satisfactory working of the striking tools isobtained, while all sparks durin the interruption of the current are avoide I will now roceed to describe my invendrawings, in which- Figure 1 is a diagram of a plant compris ing a source of alternating current, a positively driven interrupter and an electromagnetic striking tool, Figs. 2 to 5 are diagrams which will be hereinafter referred to, and

casing 1 and the magnet 2, a tool (chisel) 4 loosely insertedwith its shank in the magnet 2, a cover 5, a hammer 6 rocking about a pin 7, engaging through a hole 8 in the cover 5 and adapted to strike the upper end of the tool 4, a helical spring 9 for connecting a rear arm 10 of the hammer 6 with a lug 11 on the casingl and normally holding the hammer 6 in the detached position, and a handle 12 for pressing the tool 1 against any work-piece similar to any known pneumatic tool.

The alternating current supplied from the generator of any known construction and circulating in the circuit 13, 14 energizes by means of the coil 3 the mag et .2 forattracting the armature 6 serving as a hammer downward,'whereby a blow on the chisel 4.- 1s produced. After the blow the circuit must be opened for returning the hammer 6 to lts initial position under the action of the strained helical spring 9.

For periodically closing and opening the circuit a positively driven interrupter I of any known construction is employed. For example the interrupter I may consist of a disk 15 of insulating material fastened on a shaft 16, which turns in suitable supports 17 and is connected with the armature shaft 18 of the generator G by means of an endless chain 19 and chain wheels 20 and 21. A contact segment 22 is fastened in a suitable recess of the disk 15 and extends over a cer-' tain portion of the periphery. Two adjustable contact brushes 23 and 24 of any known construction are shown as provided on the frame (not specifically shown) of the interrupterI. 4

For the sake of clearness the electromagnetic tool T is shown to be proportionally large in relation tothe alternating current generator G and the interrupter I.

In order to better explain the method according to my invention it is necessary to first describe here certain phenomena which occur on an alternatin current being closed (wide for example Lin e, Elektrotechm'sches Archie, 1912, No. 1).

The diagram in F ig. 2 illustrates the curves of the tension E and of the strength of the current i in a purely inductive circuit permanently connected with a source of alternating current. The curve 11 is at the same time to represent the variations of the magnetic field, the current strength and the field strength being for the sake of simplicity assumed to stand in a proportion to one another. The ordinates drawn of the curves are at a distance from one another representing two thousandths of a second and consequently the ordinates correspond to a frequency of 50 cycles per second of the altermating-current. .When in such a circuit a synchronously drlven interrupter is inserted,

which always closes the circult at the instant 5 (where the tension E reaches its positive 4 current and of the field will amount to 5 and the duration of the magnetic impulse will amount to 2/100th seconds, which means that a much greater quantity of magnetic energy is now available. It would therefore be preferable to always close the circuit about at the moment where the tension is equal to zero and to again open the circuit only after about two waves of tension, that is after a full period, since the maximum of the magnetic effect will be thereby obtained.

' The increase in magnetic effect and in current produced by closing the inductive current at the moment when the tension is zero is already known and had hitherto been regarded as an unwelcome disturbance, while according to my invention it is utilized. The increase of the duration of the magnetic impulse is of special importance, since the period of 1/ 100th seconds would not be suflicient for imparting a considerable speed to a hammer of the required weight.

As can be seen from Fig. 3, at the moment 20 not only the current, but also the tension has the value zero, so that an interruption of the circuit perfectly free from sparks can be effected. I have found, that the sparkless interruption of the circuit will always be possible, when the time-interval during which the circuit is closed is nearly symmetrically situated with regard to a zeropoint (10) of the E. M. F.-curve as in this case the magnetic field is built up by one wave of tension (or a part of it) and caused to disappear by the corresponding part of the followin oppositely directed wave of tension, so t at the magnetic field and in consequence the exciting current is nearly zero at the moment of interruption.

The curve c'refers to a purely inductive circuit,"which however is impossible, sincealways a certain resistance is resent. When taking into account this resistance, the actual current will be represented by the curve -5 so'that the interruption of the current free from sparks can only take place about at the instant 18.

The'curves in both diagrams Figs. 2 and 3 relate to a circuit with constant self-induction, say to a choke-coil. When a striking tool T, such as shown in Fig. 1, is employed, than the variability of the self-induction Thecurve of the.

For this reason it-is necessary to wait'until the current (and the magnetic field) is" so "tosay sucked from the apparatus under the influence of the negative wave of tension,

and then to open the circuit, say at the point of time 17.5. A strict adherence to this moment, however, is notof special impor tance, since the strength of current between the points of time 17 and 23 is so slight, that an objectionable sparking cannot take place at the interruption of thecurrent. The long' duration of the approximately sparkless interval is a special advantage of the new method to be hereinafter described.

The apparatus cannot be operated in such a manner, that the circuit is always closed at the commencement of say the positive wave of tension and again opened shortly before the end of the negative wave, since sufiicient time must be left to the armature for returning to its initial position under the action of a spring as in Fig. 1 or of any other means, for example a second electromagnet. In order to obtain the time required for the return of the armature, I so arrange the positively or synchronously driven interrupter I, that it does not close the circuit at the commencement of every positive wave of tension, but closes only at the commencement of every second or third, etc., positive wave of tension. When for example for the fre quency of 50 the interrupter makes contact for every third period during 18/1000th seconds, then 42/1000th seconds would be left for the return of the armature, which is in accordance with practical conditions. However, the circuit may also be closed alternately at the commencement of each positive wave and at that of the next but one negative wave, so that 32/1000th seconds are left for the return. In the former case, the hammer would make =16 2/3 blows per second, and in the latter case;- %=2O blows per second. These are practical values, such as they are found with known pneumatic hammers. therefore suitable for the operation of striking tools according to the present method.

The positively driven'interrupter may be of any known construction, it may have rotary contacts similar to Fig. l or oscillatory contacts. In all cases it isonly necessary, that the moments of the closing and opening of the circuit can be adjusted, so as to avoid The usual frequency of 50 is all sparking. When the interrupter I is disposed separatel from the generator G, as 1n Flg. 1, it must be positively driven from the latter, for example by meansof a chain gear 19, 20, 21, as described, or by means of ear wheels such asthose shown at 32 and 33 in Fig.7. Or theinterrupter I may be coupled with the shaft 18'of the generator or a synchronous motor or secured on this shaft. In the latter case the number of poles of the enerator must suit the circumstances. Vhen for example the circuit is to be kept closed for one period and open for two periods, the generator G or the synchronous motor will require to have six poles such as those shown at 34 in Fig. 8 and the interrupter-disk 15 will have to make contact during one third of the revolution. When the in- .terrupterdisk 15 is driven not by the generator G but by a synchronous motor, the latter must have six poles similar to those shown at 34 in Fig. 8. For other numbers of poles the ratio of the chain wheels 20, 21 or the gear wheels 32, 33 will have to be correspondingly selected.

When the circuit is invariably closed at the commencement of a positive (or negative) wave of tension, the apparatus will be operated by means of current-impulses of the same sense. In this case a rectifier of any known construction, for example an electrolytic valve cell 25 or a mercury vapor rectifier may be employed for. switching the current on and off, so that all sparks are absolutely avoided. As the valve cell 25 would close the circuit at the commencement of each positive wave of tension and this is not always desirable, a mechanical interrupter I must-often be employed in addition to the valve cell. This interrupter should be so adjusted as to open the circuit at a time, when no current passes through, so as to avoid sparking, and to keep the circuit open as long as desired.

I have .found, that it is not always advisable to open the circuit at the moment when 11 0 (that is at the instant 17.5 in Fig. 4), since a residual magnetism may be left in the magnet coil 3 of the electromagnetic striking tool, which residual magnetism under circumstances would hold fast the armature. In such cases it is preferable to open the circuit not at the instant 17.5, but a little later, so as to permit a negative impulse to enter the magnet for canceling the residual magnetism. To this end the moment of interruption can be shifted from the point 17.5 to the point 22.5, where again i=0. ()r the circuit might be opened at the instant 17.5 (which in the case of an electrolytic rectifier is effected automatically) and the interrupter might be permanently bridged over by a comparatively high-resistance 26, which permits a slight current to pass over for canceling the residual magnetism. It is also possible to operate the electromagnetic striking tool in the de-' scribed manner without the employment of any interrupting device. It is necessary for this purpose to employ a generator of a special construction .for producing an alternatmg current corresponding to the curve of tension shown in Fig. 5. Such a generator is for instance shown in Fig. 6; The armature 27 is provided with windings 28 and 31, which are connected to the sliprings 30 and 29, from which the current is collected by brushes. As will be seen from the figure, the armature coils 28 and 31 cover only a small part of the circumference of the armature, and the field poles N and S also extend only over a small arc. When the armature rotates, a positive and a negative tension-wave will be induced in the armature-coils, when these pass under the poles, and no tension at all will be induced as long as the armature-coils rotate in the space between the poles. The relative duration of the tension-impulses and of the pauses can be selected at will by altering the angle covered by the armature-coils and by the poles.

It will now be seen, that my improved electromagnetic striking tool is operated in the same manner, whether it be connected to the special generator producing a waveshape such as shown in Fig. 5 or to an ordinary generator operating in conjunction with an interrupter such as shown in Fig. 1. In both cases I employ a succession of strong impulses (for the working stroke), weak impulses of the opposite sense (for canceling the residual magnetism) and pauses without current (for the return of the armature).

I claim:

1. The method of operating an electromagnetic striking tool by means of alternating current, which consists in intermit tently supplying current to the magnet coil during time-intervals approximately equal to one cycle, each time-interval. being symmetrically situated to one zero-value of the E. M. F. wave, so that the magnetic field for attracting the armature is not only formed, but also permitted to disappear within said time-interval.

2. The method of operating an electromagnetic striking tool by means of alternating current, which consists in connecting the tool to the supply circuit at the instant when the E. M. F. of said supply is approximately zero, maintaining the connection during a time-interval approximately equal to one cycle and interruptingthe circuit during a time-interval at least equal to one cycle.

3. .The method of operating an electromagnetic striking tool by means of alternating current, which consists in supplying to the magnet coil an alternating current the opposite sense for canceling the residual tension curve of which shows groups of one magnetism and pauses without current for positive and one negative wave and interthe return of the armature.

vals between-these groups in which the ten- LEO SCHULER. 5 sio'n is approxirnately equal to zero, so as to Witnesses: i.

' obtain strong impulses for the working WOLDEMAR HAUr'r,

stroke of the armature, weak impulses of the ARTHUR Scmzonnnn. 

